How much do polls move after Labor Day?

THERE’S A REASON that the period from Labor Day to election day is considered the home straight in a presidential contest. Robert Erikson and Christopher Wlezien, two political scientists at Columbia University and the University of Texas at Austin, have studied the history of American election-polling since 1952 and found that the leader in the polls one week after the second party convention has always won the popular vote. Two candidates have trailed in the polls around Labor Day, then won in the electoral college: George W. Bush in 2000 and Donald Trump in 2016. In both cases the polls were much tighter than they are now. Joe Biden therefore has past performance on his side.